For the Panzerzug train I also want to add a Leopold railway gun, and I've put together a basic model to add to the Sketchup model of the train. It's pretty long at around 34cm (the travelling mode of the real gun was 30m long, which scales to 344mm at 1:87 scale so the Sketchup model is pretty much spot on ), and may have trouble going around corners - although I might be able to mitigate the problems by using only 2 pairs of flanged wheels in each bogie, so that they act as a pair of long wheelbase railcars at each end supporting the gun carriage.

And a couple of "sketches" while messing with the Sketchup display options: